In a known television scrambling system described in U.S. Pat. No. 4,074,311 (Tanner et al.), a scrambling signal is injected into the television signal between the video and audio carriers. The signal is removed by a notch filter for decoding.
Tanner et al. also teach signal preemphasis centered on the frequency of the scrambling signal to compensate for signal degradation due to the removal of the scrambling signal by the notch filter signal during decoding.
The Tanner system suffers from several disadvantages. For example, the signal preemphasis is generally inadequate to compensate for the degradation which results from the removal of the scrambling signal, particularly when the picture includes high spatial frequency components which require a high spectral content of the transmitted signal. This degradation is manifested by a blurring of the received picture which in some cases, for example, makes it difficult to read textual parts of the picture.